Centrifugal compressor

ABSTRACT

A centrifugal compressor includes a casing, a first compression mechanism and a second compression mechanism. The casing has a first inlet portion, a first outlet portion, a second inlet portion and a second outlet portion. The first compression mechanism includes a first inlet guide vane disposed in the first inlet portion, a first impeller disposed downstream of the first inlet guide vane, a first diffuser disposed in the first outlet portion downstream from the first impeller, and a first motor. The second compression mechanism includes a second inlet guide vane disposed in the second inlet portion, a second impeller disposed downstream of the second inlet guide vane, a second diffuser disposed in the second outlet portion downstream from the second impeller, and a second motor. The first and second motors are arranged to rotate first and second shafts in order to rotate the first and second impellers.

BACKGROUND Field of the Invention

The present invention generally relates to a centrifugal compressor. More specifically, the present invention relates to a centrifugal compressor with a pair of compressors and a pair of motors.

Background Information

A chiller system is a refrigerating machine or apparatus that removes heat from a medium. Commonly a liquid such as water is used as the medium and the chiller system operates in a vapor-compression refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. A conventional chiller system often utilizes a centrifugal compressor, which is often referred to as a turbo compressor. Thus, such chiller systems can be referred to as turbo chillers. Alternatively, other types of compressors, e.g. a screw compressor, can be utilized.

In a conventional (turbo) chiller, refrigerant is compressed in the centrifugal compressor and sent to a heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as a condenser because the refrigerant condenses in this heat exchanger. As a result, heat is transferred to the medium (liquid) so that the medium is heated. Refrigerant exiting the condenser is expanded by an expansion valve and sent to another heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as an evaporator because refrigerant is heated (evaporated) in this heat exchanger. As a result, heat is transferred from the medium (liquid) to the refrigerant, and the liquid is chilled. The refrigerant from the evaporator is then returned to the centrifugal compressor and the cycle is repeated. The liquid utilized is often water.

A conventional centrifugal compressor basically includes a casing, an inlet guide vane, an impeller, a diffuser, a motor, various sensors and a controller. Refrigerant flows in order through the inlet guide vane, the impeller and the diffuser. Thus, the inlet guide vane is coupled to a gas intake port of the centrifugal compressor while the diffuser is coupled to a gas outlet port of the impeller. The inlet guide vane controls the flow rate of refrigerant gas into the impeller. The impeller increases the velocity of refrigerant gas. The diffuser works to transform the velocity of refrigerant gas (dynamic pressure), given by the impeller, into (static) pressure. The motor rotates the impeller. The controller controls the motor, the inlet guide vane and the expansion valve. In this manner, the refrigerant is compressed in a conventional centrifugal compressor. A conventional centrifugal compressor may have one or two stages. A motor drives the one or more impellers.

See U.S. Pat. No. 7,942,628 and U.S. Patent Application publication No. 2010/0251750 as examples of conventional technology.

SUMMARY

The chiller industry began to offer variable speed compressors during the 1990's for improved efficiency.

The chiller industry has also developed 2-stage type centrifugal compressors for higher chiller efficiency.

In the case of a two stage centrifugal compressor structure, one motor is used to drive both impellers. It has been discovered that this structure has issues with (1) operating range and (2) efficiency.

Regarding (1) Operating range, it has been discovered that there is relationship between operating range and rotational speed in each impeller. Since the current technology only allows each impeller to rotate at the same speed as the other, it has been discovered that the compressor will be difficult and/or impossible to operate when it is attempted to operate either impeller outside the range.

Regarding (2) Efficiency, it has been discovered that once either impeller does not operate at the point designed, the efficiency of compressor will decline.

Regarding (1) Operating range, it has been discovered that by rotating each impeller in different rotational speed, the impellers may not be operated outside of their operating ranges (i.e., may be maintained within their operating ranges).

Regarding (2) Efficiency, it has been discovered that 1^(st) and 2^(nd) stage impeller's rotational speed may be adjusted to increase their efficiency, and it may therefore be possible to improve overall compressor efficiency.

For current technology's operation, each compressor's operating range will be dominated by impeller's operating range. A two stage compressor can have more limited operating range capability due to the fact that each stage can have its own boundary limits (choke flow limits, stall and surge limit, minimum unloading limit). Therefore, the compressor cannot or should not be operated when either impeller operates outside of the range.

For current technology's operation, compressor efficiency will decline if either impeller does not operate at its designed point. The reason for this is due to the change of head coefficient and flow coefficient. Once these values are changed, the compressor cannot operate at its designed (Highest efficiency) point.

In addition, it has been discovered that two stage cycle efficiency advantage over one stage is reduced or mitigated when operating away from the design point if the two stages are not well matched, causing more severe efficiency reduction when moving away from the design point and the peak efficiency point. It has been discovered that his can result in chillers spending a significant proportion of operating time away from the “full load design point”.

Therefore one object of the present invention is to provide a centrifugal compressor that can maintain operation within the operating range.

Another object of the present invention is to provide a centrifugal compressor that can improve efficiency.

Yet another object of the present invention is to provide a centrifugal compressor that addresses one or more of the other discovered problems mentioned above or already known to those skilled in the art.

One or more of the foregoing objects can basically be achieved by providing a centrifugal compressor including a casing, a first compression mechanism and a second compression mechanism. The casing has a first inlet portion, a first outlet portion, a second inlet portion and a second outlet portion. The first compression mechanism includes a first inlet guide vane disposed in the first inlet portion, a first impeller disposed downstream of the first inlet guide vane, a first diffuser disposed in the first outlet portion downstream from the first impeller, and a first motor. The first impeller is attached to a first shaft rotatable about a first rotation axis. The first motor is arranged to rotate the first shaft in order to rotate the first impeller. The second compression mechanism includes a second inlet guide vane disposed in the second inlet portion, a second impeller disposed downstream of the second inlet guide vane, a second diffuser disposed in the second outlet portion downstream from the second impeller, and a second motor. The second impeller is attached to a second shaft rotatable about a second rotation axis. The second motor is arranged to rotate the second shaft in order to rotate the second impeller.

These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of this original disclosure:

FIG. 1 is a schematic diagram illustrating a two stage chiller system (with an economizer) having a centrifugal compressor in accordance with an embodiment of the present invention;

FIG. 2 is a perspective view of the centrifugal compressor of the chiller system illustrated in FIG. 1, with portions broken away and shown in cross-section for the purpose of illustration;

FIG. 3 is a perspective view of internal parts (e.g., shafts, impellers, magnetic bearings and motors) of the centrifugal compressor illustrated in FIGS. 1-2;

FIG. 4 is an elevational view of the internal parts (e.g., shafts, impellers, magnetic bearings and motors) of the centrifugal compressor illustrated in FIG. 3;

FIG. 5 is a schematic longitudinal partial cross-sectional view of the internal parts (e.g., shafts, impellers, magnetic bearings and motors) of the centrifugal compressor illustrated in FIGS. 3-4, with additional parts such as sensors, back-up bearings, coils, etc. schematically illustrated in more detail;

FIG. 6 is a flow chart illustrating a normal control of the centrifugal compressor illustrated in FIGS. 1-5;

FIG. 7A is a graph illustrating operating range of a two stage compressor (overall compressor operation), with A representing an overall operating point outside the overall operating range;

FIG. 7B is a graph illustrating operating range of a first stage impeller of the two stage compressor illustrated in FIG. 7A, with A1 representing a first stage operating point outside the first stage operating range;

FIG. 7C is a graph illustrating operating range of a second stage impeller of the two stage compressor illustrated in FIG. 7A, with A2 representing a second stage operating point inside the second stage operating range;

FIG. 8A is a graph illustrating operating range of a two stage compressor (overall compressor operation), with A representing an overall operating point outside the overall operating range (like FIG. 7A) and with B representing a shifted operating point within the overall operating range in accordance with the present invention;

FIG. 8B is a graph illustrating operating range of a first stage impeller of the two stage compressor illustrated in FIG. 8A, with A1 representing a first stage operating point outside the first stage operating range (like FIG. 7B) and with B1 representing a shifted first operating point by decreasing the rotation speed of the first stage impeller in accordance with the present invention;

FIG. 8C is a graph illustrating operating range of a second stage impeller of the two stage compressor illustrated in FIG. 7A, with A2 representing a second stage operating point inside the second stage operating range (like FIG. 7C);

FIG. 9A is a graph illustrating efficiency of a two stage compressor (overall compressor efficiency), with E representing a designed highest efficiency point and with D and E representing shifted lower efficiency operating points;

FIG. 9B is a graph illustrating efficiency of a first stage impeller of the two stage compressor illustrated in FIG. 9A, with E1 representing a designed highest efficiency point of the first stage and with D1 and E1 representing shifted lower efficiency operating points of the first stage;

FIG. 9C is a graph illustrating efficiency of a second stage impeller of the two stage compressor illustrated in FIG. 9A, with E2 representing a designed highest efficiency point of the second stage and with D2 and E2 representing shifted lower efficiency operating points of the second stage;

FIG. 10A is a graph illustrating efficiency of a two stage compressor (overall compressor efficiency) like FIG. 9A, with E representing a designed highest efficiency point and with D and E representing shifted lower efficiency operating points;

FIG. 10B is a graph illustrating efficiency of a first stage impeller of the two stage compressor illustrated in FIG. 9A, with E1 representing a designed highest efficiency point of the first stage and with D1 and F1 representing shifted lower efficiency operating points of the first stage, and with arrows illustrating how the efficiency can be increased from points D1 or F1 by reducing or increasing the first impeller speed, respectively; and

FIG. 10C is a graph illustrating efficiency of a second stage impeller of the two stage compressor illustrated in FIG. 9A, with E2 representing a designed highest efficiency point of the second stage and with D2 and F2 representing shifted lower efficiency operating points of the second stage, and with arrows illustrating how the efficiency can be increased from points D2 or F2 by reducing or increasing the second impeller speed, respectively.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Selected embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

Referring initially to FIG. 1, a chiller system 10 having a centrifugal compressor 22 in accordance with an embodiment of the present invention is illustrated. The centrifugal compressor 22 of FIG. 1 is a two stage compressor, and thus, the chiller system 10 of FIG. 1 is a two stage chiller system. The two stage chiller system of FIG. 1 also includes an economizer. FIG. 1 merely illustrates one example of a chiller system in which the centrifugal compressor 22 in accordance with the present invention can be used. For example, it will be apparent to those skilled in the art from this disclosure that the economizer of the chiller system 10 can be eliminated. However, in the illustrated embodiment the economizer is present for the reasons discussed below.

The chiller system 10 is conventional, except for the centrifugal compressor 22 and the manner in which the centrifugal compressor 22 is controlled. Specifically, in the illustrated embodiment, the centrifugal compressor 22 includes a structure and is controlled to maintain operation within its operating range and to improve efficiency, as explained in more detail below. Therefore the chiller system 10 will not be discussed and/or illustrated in detail herein except as related to the centrifugal compressor 22 the manner in which the centrifugal compressor 22 is controlled. However, it will be apparent to those skilled in the art that the conventional parts of the chiller system 10 can be constructed in variety of ways without departing the scope of the present invention. In the illustrated embodiments, the chiller system 10 is preferably a water chiller that utilizes cooling water and chiller water in a conventional manner.

Referring still to FIG. 1, the components of the chiller system 10 will now briefly be explained. The chiller system 10 basically includes a chiller controller 20, the centrifugal compressor 22, a condenser 24, an expansion valve or orifice 25, an economizer 26, an expansion valve or orifice 27, and an evaporator 28 connected together in series to form a loop refrigeration cycle. The economizer 26 is connected between the first compression mechanism and the second compression mechanism in the refrigerant circuit (e.g., to the intermediate stage). Various sensors (not shown) are disposed throughout the circuit of the chiller system 10. Such sensors and use of information from such sensors to control the chiller system 10 is conventional, and thus, will not be explained and/or illustrated in detail herein except as related to controlling the centrifugal compressor 22 in accordance with the present invention. Therefore, it will be apparent to those skilled in the art from this disclosure that normal operation of the chiller system 10 will be omitted for the sake of brevity, except as related the structure and operation of the centrifugal compressor 22.

Referring now to FIGS. 1-5, the compressor 22 will now be explained in more detail. The compressor 22 is a two-stage centrifugal compressor in the illustrated embodiment. Thus, the compressor 22 illustrated herein includes two impellers. However, the compressor 22 may include three or more impellers (not shown). The two-stage centrifugal compressor 22 of the illustrated embodiment is conventional except that the compressor 22 includes separate motors used to drive the two impellers. In addition, the motors are controlled in accordance with the present invention.

Thus, the centrifugal compressor 22 includes first and second stages fluidly connected in series so that refrigerant compressed in the first stage is subsequently compressed in the second stage. The first stage includes a first stage inlet guide vane 32 a, a first impeller 34 a, a first diffuser/volute 36 a, a first stage compressor motor 38 a and a first stage magnetic bearing 40 a. Similarly, the second stage includes a second stage inlet guide vane 32 b, a second impeller 34 b, a second diffuser/volute 36 b, a second stage compressor motor 38 b and a second stage magnetic bearing 40 b. In addition, the centrifugal compressor 22 includes various conventional sensors (only some shown).

While magnetic bearings are described herein, it will be apparent to those skilled in the art from this disclosure that other types and forms of compressor bearings maybe used with this invention. A casing 30 covers the other parts of the centrifugal compressor 22. The casing 30 includes an inlet portion 31 a and an outlet portion 33 a for the first stage of the compressor 22. The casing 30 also includes an inlet portion 31 b and an outlet portion 33 b for the second stage of the compressor 22. Thus, the casing 30 has a first inlet portion 31 a, a first outlet portion 33 a, a second inlet portion 31 b and a second outlet portion 33 b.

Thus, in the illustrated embodiment, the centrifugal compressor 22 includes the casing 30, a first compression mechanism (first stage) 23 a and a second compression mechanism (second stage) 23 b. The first compression mechanism 23 a includes the first inlet guide vane 32 a disposed in the first inlet portion 31 a, the first impeller 34 a disposed downstream of the first inlet guide vane 32 a, a first diffuser 36 a disposed in the first outlet portion 33 a downstream from the first impeller 34 a, and the first motor 38 a arranged to rotate the first shaft 42 a in order to rotate the first impeller 34 a. The first impeller 34 a is attached to a first shaft 42 a rotatable about a first rotation axis X₁. The second compression mechanism 23 b includes the second inlet guide vane 23 b disposed in the second inlet portion 31 b, the second impeller 34 b disposed downstream of the second inlet guide vane 32 b, the second diffuser 36 b disposed in the second outlet portion 33 b downstream from the second impeller 34 b, and the second motor 38 b arranged to rotate the second shaft 42 b in order to rotate the second impeller 34 b. The second impeller 34 b is attached to a second shaft 42 b rotatable about a second rotation axis X₂.

The chiller controller 20 receives signals from the various sensors and controls the inlet guide vanes 32 a and 32 b, the compressor motors 38 a and 38 b, and the magnetic bearings 40 a and 40 b, as explained in more detail below. Refrigerant flows in order through the first stage inlet guide vane 32 a, the first stage impeller 34 a, the second stage inlet guide vane 32 b, and the second stage impeller 34 b. The inlet guide vanes 32 a and 32 b control the flow rate of refrigerant gas into the impellers 34 a and 34 b, respectively, in a conventional manner. The impellers 34 a and 34 b increase the velocity of refrigerant gas, generally without changing pressure. The motor speeds determine the amount of increase of the velocity of refrigerant gas. The diffusers/volutes 36 a and 36 b increase the refrigerant pressure.

The diffusers/volutes 36 a and 36 b are non-movably fixed relative to the casing 30. The compressor motors 38 a and 38 b rotate the impellers 34 a and 34 b via first and second shafts 42 a and 42 b, respectively. The first diffuser 36 a is connected to the second impeller 34 b such that refrigerant compressed in the first compression mechanism 23 a is further compressed in the second compression mechanism 23 b, as best understood from FIG. 1. The first and second magnetic bearings 40 a and 40 b magnetically rotatable support the shafts 42 a and 42 b, respectively. Alternatively, the bearing system may include a roller element, a hydrodynamic bearing, a hydrostatic bearing, and/or a magnetic bearing, or any combination of these. In this manner, the refrigerant is compressed in the centrifugal compressor 22.

In operation of the chiller system 10, the first stage impeller 34 a and the second stage impeller 34 b of the compressor 22 are rotated, and the refrigerant of low pressure in the chiller system 10 is sucked by the first stage impeller 34 a. The flow rate of the refrigerant is adjusted by the inlet guide vane 32 a. The refrigerant sucked by the first stage impeller 34 a is compressed to intermediate pressure, the refrigerant pressure is increased by the first diffuser/volute 36 a, and the refrigerant is then introduced to the second stage impeller 34 b. The flow rate of the refrigerant is adjusted by the inlet guide vane 32 b. The second stage impeller 34 b compresses the refrigerant of intermediate pressure to high pressure, and the refrigerant pressure is increased by the second diffuser/volute 36 b. The high pressure gas refrigerant is then discharged to the chiller system 10. In the illustrated embodiment, because the impellers 34 a and 34 b are driven by separate motors 38 a or 38 b, the rotation speeds of the first and second impellers 34 a and 34 b are independently variable.

Referring to FIGS. 2-5, the first and second magnetic bearings 40 a and 40 b will now be explained in more detail. The first and second magnetic bearings 40 a and 40 b are conventional, except as explained herein. Thus, the first and second magnetic bearings 40 a and 40 b will not be discussed and/or illustrated in detail herein, except as related to the present invention. Rather, it will be apparent to those skilled in the art that any suitable magnetic bearing can be used without departing from the present invention. The first magnetic bearing 40 a preferably includes a first impeller side radial magnetic bearing 44 a, a first remote side radial magnetic bearing 46 a and a first axial (thrust) magnetic bearing 48 a. Similarly, the second magnetic bearing 40 b preferably includes a second impeller side radial magnetic bearing 44 b, a second remote side radial magnetic bearing 46 b and a second axial (thrust) magnetic bearing 48 b.

At least one radial magnetic bearing 44 a or 46 a rotatably supports the first shaft 42 a, and at least one radial magnetic bearing 44 b or 46 b rotatably supports the second shaft 42 b. The thrust magnetic bearing 48 a axially supports the first shaft 42 a along a first rotational axis X₁ by acting on a first thrust disk 45 a. The thrust magnetic bearing 48 a includes the thrust disk 45 a which is attached to the first shaft 42 a. Similarly, the thrust magnetic bearing 48 b axially supports the second shaft 42 b along a second rotational axis X₂ by acting on a second thrust disk 45 b. The thrust magnetic bearing 48 b includes the thrust disk 45 b which is attached to the second shaft 42 b.

The first thrust disk 45 a extends radially from the first shaft 42 a in a direction perpendicular to the first rotational axis X₁, and is fixed relative to the first shaft 42 a. The second thrust disk 45 b extends radially from the second shaft 42 b in a direction perpendicular to the second rotational axis X₂, and is fixed relative to the second shaft 42 b. A position of the first shaft 42 a along the first rotational axis X₁ (an axial position) is controlled by an axial position of the first thrust disk 45 a. Likewise, a position of the second shaft 42 b along the first rotational axis X₂ (an axial position) is controlled by an axial position of the second thrust disk 45 b. The first radial magnetic bearings 44 a and 46 a are disposed on opposite axial ends of the first compressor motor 38 a, while the second radial magnetic bearings 44 b and 46 b are disposed on opposite axial ends of the second compressor motor 38 b. In the illustrated embodiment, the first and second rotation axes X₁ and X₂ are coincident with each other. Moreover, in the illustrated embodiment, the first and second rotation axes X₁ and X₂ are parallel.

Referring still to FIGS. 2-5, various sensors detect radial and axial positions of the shafts 42 a and 42 b relative to the magnetic bearings 44 a, 44 b, 46 a, 46 b, 48 a and 48 b, and send signals to the chiller controller 20 in a conventional manner. The chiller controller 20 then controls the electrical current sent to the magnetic bearings the magnetic bearings 44 a, 44 b, 46 a, 46 b, 48 a and 48 b in a conventional manner to maintain the shafts 42 a and 42 in the correct positions. Thus, the magnetic bearing 40 a is preferably a combination of active magnetic bearings 44 a, 46 a, and 48 a, which utilizes gap sensors 54 a, 56 a and 58 a (FIG. 5) to monitor shaft position and send signals indicative of shaft position to the chiller controller 20. Thus, each of the magnetic bearings 44 a, 46 a and 48 a are preferably active magnetic bearings. Likewise, the magnetic bearing 40 b is preferably a combination of active magnetic bearings 44 b, 46 b, and 48 b, which utilizes gap sensors 54 b, 56 b and 58 b (FIG. 5) to monitor shaft position and send signals indicative of shaft position to the chiller controller 20. Thus, each of the magnetic bearings 44 b, 46 b and 48 b are preferably active magnetic bearings.

Therefore, the centrifugal compressor 22 includes a first magnetic bearing 40 a rotatably supporting the first shaft 42 a, and a second magnetic bearing 40 b rotatably supporting the second shaft 42 b. The first shaft 42 a has a first inlet end with the first impeller 34 a mounted thereon and a first remote end with the first motor 38 a mounted on the first shaft 42 a between the first impeller 34 a and the first remote end, and the second shaft 42 b has a second inlet end with the second impeller 34 b mounted thereon and a second remote end with the second motor 38 b mounted on the second shaft 42 b between the second impeller 34 b and the second remote end.

As mentioned above, the first and second magnetic bearings 40 a and 40 b include a combination of radial and axial magnetic bearings. Specifically, the magnetic bearing 44 a is the first impeller side radial magnetic bearing axially disposed between the first impeller 34 a and the first motor 38, and the magnetic bearing 44 b is the second impeller side radial magnetic bearing axially disposed between the second impeller 34 b and the second motor 38 b. The magnetic bearing 46 a is the first remote side radial magnetic bearing axially disposed on a side of the first motor 38 a that is remote from a side where the first impeller 34 a is mounted, and the magnetic bearing 46 b is the second remote side radial magnetic bearing axially disposed on a side of the second motor 38 b that is remote from a side where the second impeller 34 b is mounted. In any case, the first magnetic bearing 40 a includes at least one first radial magnetic bearing 44 a or 44 b and at least one first axial thrust magnetic bearing 48 a, and the second magnetic bearing 40 b includes at least one second radial magnetic bearing 44 b or 46 b and at least one second axial thrust magnetic bearing 48 b.

In the illustrated embodiment, the first axial thrust magnetic bearing 48 a is axially disposed adjacent to the first remote side radial bearing 46 a, and the second axial thrust magnetic bearing 48 b is axially disposed adjacent to the second remote side radial bearing 46 b. Thus, the first axial thrust magnetic bearing 48 a is axially disposed at the first remote end of the first shaft 42 a, and the second axial thrust magnetic bearing 48 b is axially disposed at the second remote end of the second shaft 42 b. In addition, the first and second remote ends (of the shafts 42 a and 42 b) and the first and second axial thrust magnetic bearings 48 a and 48 b are axially spaced from each other to form a gap therebetween.

The gap sensors 54 a, 54 b, 56 a, 56 b, 58 a and 58 b are only shown schematically in FIG. 5. Likewise, back-up bearings (unnumbered), which are located at each end of each shaft 42 a and 42 b are provided in the illustrated embodiment as only shown in FIG. 5. It will be apparent to those skilled in the art from this disclosure that the back-up bearings (unnumbered) could be eliminated. Likewise, it will be apparent to those skilled in the art from this disclosure that one or more of the gap sensors could be eliminated in order to simplify the magnetic bearings 40 a and 40 b. Moreover, it will be apparent to those skilled in the art from this disclosure that if the gap sensors are illuminated, the magnetic bearings could be controlled passively by the chiller controller 20.

Referring to FIGS. 1-5, the motors 38 a and 38 b in accordance with the present invention will now be explained in more detail. The first motor 38 a includes a first stator 60 a and a first rotor 62 a. Likewise, the second motor 38 b includes a second stator 60 b and a second rotor 62 b. The stator 60 a is fixed to an interior surface of the casing 30, while the rotor 62 a is fixed to the shaft 42 a. Likewise, the stator 60 b is fixed to an interior surface of the casing 30, while the rotor 62 b is fixed to the shaft 42 b. The stators 60 a and 60 b and the rotors 62 a and 62 b are conventional. Thus, when electricity is sent to the stator 60 a, the rotor 62 a is caused to rotate at a speed according to the supplied electricity. Moreover, when electricity is sent to the stator 60 b, the rotor 62 b is caused to rotate at a speed according to the supplied electricity. Since the rotors are fixed to the shafts, the shafts are also caused to rotate, and thus, the impellers 34 a and 34 b are also cause to rotate.

As mentioned above, because two separate motors 38 a and 38 b are provided to rotate the first and second impellers 34 a and 34 b, the first and second impellers 34 a and 34 b can be rotated independently at different speeds. More specifically, the motors 38 a and 38 b preferably receive electricity from separate Variable Frequency Drives (VFDs) 64 a and 64 b, respectively. The Variable Frequency Drives (VFDs) 64 a and 64 b receive control signals from the chiller controller 20 to independently control the speeds of rotation of the first and second impellers 34 a and 34 b, respectively. The manner in which the Variable Frequency Drives (VFDs) 64 a and 64 b are controlled will be explained below with reference to the control flow chart illustrated in FIG. 6 and the graphs illustrated in FIGS. 7A-10C.

Referring now to FIG. 6, independent control of the motors 38 a and 38 b to independently control the rotations speeds of the first and second impellers 34 a and 34 b will now be explained in more detail. As mentioned above, rotation speeds of the first and second motors 38 a and 38 b are independently controllable. Specifically, the controller 20 is programmed to independently control the rotation speeds of the first and second motors 38 a and 38 b in accordance with the flow chart of FIG. 6. This loop illustrated in FIG. 6 starts and repeats with following triggers: (1.) Compressor discharge pressure changes more than 10%/min; and/or (2.) Compressor suction pressure changes more than 10%/min. However, if the customer changes setting of chiller (i.e., the leaving/exiting water temperature setting), this could be trigger of FIG. 6 too. The first Variable Frequency Drive (VFD) 64 a is connected to the first motor 38 a and the controller 20 to variably control the rotation speed of the first motor 38 a in accordance with FIG. 6, and the second Variable Frequency Drive (VFD) 64 b is connected to the second motor 38 b and the controller 20 to variably control the rotation speed of the second motor 38 b in accordance with FIG. 6.

The start/repeat point of FIG. 6 is at step S1, while the finish/repeat point is at step S14. Steps S2-S4 are steps used to calculate current efficiency and determine if the first stage compression mechanism 23 a is operating at a most efficient point (See for Example FIG. 9B). If it is determined that the first stage compression mechanism 23 a is operating at a most efficient point at step S4, the controller 20 proceeds to step S5. If not, the controller 20 proceeds to step S8. In steps S8-S10, the controller 20 adjusts the inlet guide vane 32 a and the first VFD speed to improve the efficiency of the first compression mechanism 23 a (See for example FIG. 10B). If it is determined that the first stage compression mechanism 23 a is operating at a most efficient point after these changes at step S10, the controller 20 proceeds to step S5. If not, the controller 20 returns to *A to repeat the above determinations and controls.

When the controller 20 has proceeded to step S5, the same logic as the preceding paragraph is repeated for the second stage compressions mechanism. Steps S5-S7 are steps used to calculate current efficiency and determine if the second stage compression mechanism 23 b is operating at a most efficient point (See for Example FIG. 9C). If it is determined that the second stage compression mechanism 23 b is operating at a most efficient point at step S7, the controller 20 proceeds to step S14. If not, the controller 20 proceeds to step S11. In steps S11-S13, the controller 20 adjusts the inlet guide vane 32 b and the second VFD speed to improve the efficiency of the second compression mechanism 23 b (See for example FIG. 10C). If it is determined that the second stage compression mechanism 23 b is operating at a most efficient point after these changes at step S13, the controller 20 proceeds to step S14. If not, the controller 20 returns to *B to repeat the above determinations and controls.

In addition to the control illustrated in FIG. 6, the rotation speeds of the VFDs 64 a and 64 b can be controlled to maintain the first and second compression mechanisms 23 a and 23 b in their operating ranges. However, this will not be illustrated in a flow chart since the logic of FIG. 6 can be used, except “efficiency” is replaced with “operating range.” This type of control can also be understood from FIGS. 7A-8C, which will be explained below.

More details of operation of the centrifugal compressor in accordance with the present invention will now be discussed. In the illustrated embodiment, the two stages are separate so that each stage (each impeller) operates by independent speed control. By separately varying the speed of each impeller the operation range of each impeller can be maintained within its boundary limits as mentioned in the preceding paragraph. In addition varying economizer flow over wide range of operating conditions can be possible when speeds of the impellers are independently variable as disclosed herein. Furthermore, because each stage (each impeller) is operable by independent speed control; by separately varying the speed of each impeller the boundary limits can be adjusted for better matching of the each of the stages, and an increase in the operating range of the two stage compressor; independent speed control allows for better balancing of mass flow and work input between the each stages, especially when considering varying economizer flow over wide range of operating conditions.

The most restrictive operating boundary limit from either stage becomes the limit of the 2-stage compressor; so impeller matching (selection of compatible impellers) can become important for a configurable product that can be sold to many different customers with many different operating conditions; poor matching results in useless operating range (it works very well at or near the design point but cannot operate well away from the design point, or can operate but the efficiency & cost is not competitive against single-stage design); even best case scenario for impeller matching could show improved operating range by applying this new concept; across a wide range of operating conditions, the side flow from economizer vapor into the inlet of the second stage creates a significant design challenge to find an “optimal” design because the second stage impeller mass flow varies quite a lot.

There is relationship between operating range and rotational speed in each impeller. Since current technology (a normal 2 stage compressor with one motor and two impellers) only allows each impeller to rotate at the same speed, the compressor will be impossible to operate when either impeller operates at outside the range. In addition, with current technology (a normal 2 stage compressor with one motor and two impellers) once either impeller does not operate at the point designed, the efficiency of compressor will be dropped.

The illustrated embodiment technology can improve compressor's operating range and efficiency because new structure allows each compressor to rotate in different speed. Specifically, by rotating each impeller in different rotational speed, impellers would not be operated at outside of the operating range. Also, efficiency of 1^(st) and 2^(nd) stage impeller's rotational speed will be adjusted to increase their efficiency, and it will improve overall compressor efficiency.

FIG. 7A is a graph illustrating operating range of a two stage compressor (overall compressor operation), with A representing an overall operating point outside the overall operating range. FIG. 7B is a graph illustrating operating range of the first stage impeller, with A1 representing a first stage operating point outside the first stage operating range. FIG. 7C is a graph illustrating operating range of the second stage impeller, with A2 representing a second stage operating point inside the second stage operating range;

For current technology's operation (a normal 2 stage compressor with one motor and two impellers), each compressor's operating range will be dominated by impeller's operating range. Therefore, compressor cannot be operated when either impeller operates at outside of the range. As FIGS. 7A-7C show, the 2^(nd) stage impeller can be operated at A2 (FIG. 7C), but 1^(st) stage impeller cannot be operated at A1 (FIG. 7B). As a result, compressor will not be operated at A (FIG. 7A).

FIG. 8A is a graph illustrating operating range of a two stage compressor (overall compressor operation), with A representing an overall operating point outside the overall operating range (like FIG. 7A) and with B representing a shifted operating point within the overall operating range in accordance with the present invention. In FIG. 8B, A1 represents a first stage operating point outside the first stage operating range (like FIG. 7B) and B1 represents a shifted first operating point by decreasing the rotation speed of the first stage impeller in accordance with the present invention. FIG. 8C is like FIG. 7C where A2 represents a second stage operating point inside the second stage operating range.

By rotating each impeller at a different speed, the impellers can both be operated inside the range. As FIG. 8B shows the 1^(st) stage impeller's operating point will be moved from A1 to B1 by decreasing the rotational speed. As a result, the overall compressor's operating point will be moved from A to B in FIG. 8A. The 2^(nd) stage impeller's operating point will not be changed since it operates at inside the range already.

FIG. 9A is a graph illustrating efficiency of a two stage compressor (overall compressor efficiency), with E representing a designed highest efficiency point and with D and E representing shifted lower efficiency operating points. In FIG. 9B, E1 represents a designed highest efficiency point of the first stage and D1 and E1 represent shifted lower efficiency operating points of the first stage. Likewise, in FIG. 9C, with E2 represents a designed highest efficiency point of the second stage and D2 and E2 represent shifted lower efficiency operating points of the second stage.

For current technology's operation (a normal 2 stage compressor with one motor and two impellers), compressor efficiency will be dropped if either impeller does not operate at designed point. The reason for this is due to the change of head coefficient and flow coefficient. Once these values are changed, the compressor cannot operate at designed (Highest efficiency) point. See FIGS. 9A-9C.

FIG. 10A is a graph illustrating efficiency of a two stage compressor (overall compressor efficiency) like FIG. 9A, with E representing a designed highest efficiency point and with D and E representing shifted lower efficiency operating points. In FIG. 10B, E1 represents a designed highest efficiency point of the first stage and D1 and F1 represent shifted lower efficiency operating points of the first stage. The arrows illustrate how the efficiency can be increased from points D1 or F1 by reducing or increasing the first impeller speed, respectively. FIG. 10C is the same as FIG. 10b but for the second stage.

By rotating each impeller in different speed, each impeller can be operated at the point close to the designed point. When flow coefficient is low (point like D1 or D2), the impeller speed will be decreased to get higher efficiency. On the other hand, when flow coefficient is high (point like F1 and F2), the impeller speed will be increased to get higher efficiency. The overall compressor efficiency will be close to the highest efficiency. The change of rotation speed will require the change of IGV position as well. This is because the flow coefficient and head coefficient has been changed due to the rotational speed change. Specifically, if RPM of an impeller increases then the IGV should close to reduce inlet flow. On the other hand, if RPM of an impeller decreases then the IGV should open to increase inlet flow.

Referring to FIGS. 1-6, the chiller controller 20 may include numerous control sections programmed to control the conventional parts in a conventional manner. For example, conventional magnetic bearing control sections, conventional compressor variable frequency drives, a conventional compressor motor control sections, conventional inlet guide vane control sections, and conventional expansion valve control sections. These sections can be separate or combined sections.

In the illustrated embodiment, the control sections are sections of the chiller controller 20 programmed to execute the control of the parts of the chiller 10 in accordance with FIG. 6 and as described and illustrated herein. However, it will be apparent to those skilled in the art from this disclosure that the precise number, location and/or structure of the control sections, portions and/or chiller controller 20 can be changed without departing from the present invention so long as the one or more controllers are programed to execute control of the parts of the chiller system 10 as explained herein.

The chiller controller 20 is conventional, and thus, includes at least one microprocessor or CPU, an Input/output (I/O) interface, Random Access Memory (RAM), Read Only Memory (ROM), a storage device (either temporary or permanent) forming a computer readable medium programmed to execute one or more control programs to control the chiller system 10 as disclosed herein. The chiller controller 20 may optionally include an input interface such as a keypad to receive inputs from a user and a display device used to display various parameters to a user. The parts and programming are conventional, except as explained herein, and thus, will not be discussed in further detail herein, except as needed to understand the embodiment(s).

General Interpretation of Terms

In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts.

The term “detect” as used herein to describe an operation or function carried out by a component, a section, a device or the like includes a component, a section, a device or the like that does not require physical detection, but rather includes determining, measuring, modeling, predicting or computing or the like to carry out the operation or function.

The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function.

The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed.

While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A centrifugal compressor comprising: a casing having a first inlet portion, a first outlet portion, a second inlet portion and a second outlet portion; a first compression mechanism including a first inlet guide vane disposed in the first inlet portion, a first impeller disposed downstream of the first inlet guide vane, the first impeller being attached to a first shaft rotatable about a first rotation axis, a first diffuser disposed in the first outlet portion downstream from the first impeller, and a first motor arranged to rotate the first shaft in order to rotate the first impeller; and a second compression mechanism including a second inlet guide vane disposed in the second inlet portion, a second impeller disposed downstream of the second inlet guide vane, the second impeller being attached to a second shaft rotatable about a second rotation axis, a second diffuser disposed in the second outlet portion downstream from the second impeller, and a second motor arranged to rotate the second shaft in order to rotate the second impeller.
 2. The centrifugal compressor according to claim 1, wherein rotation speeds of the first and second motors are independently controllable.
 3. The centrifugal compressor according to claim 2, further comprising a controller programmed to independently control the rotation speeds of the first and second motors.
 4. The centrifugal compressor according to claim 3, further comprising a first Variable Frequency Drive (VFD) connected to the first motor and the controller to variably control the rotation speed of the first motor; and a second Variable Frequency Drive (VFD) connected to the second motor and the controller to variably control the rotation speed of the second motor.
 5. The centrifugal compressor according to claim 1, further comprising a first magnetic bearing rotatably supporting the first shaft; and a second magnetic bearing rotatably supporting the second shaft.
 6. The centrifugal compressor according to claim 5, wherein the first and second rotation axes are coincident with each other.
 7. The centrifugal compressor according to claim 6, wherein the first shaft has a first inlet end with the first impeller mounted thereon and a first remote end with the first motor mounted on the first shaft between the first impeller and the first remote end, the second shaft has a second inlet end with the second impeller mounted thereon and a second remote end with the second motor mounted on the second shaft between the second impeller and the second remote end, the first magnetic bearing includes a first impeller side radial magnetic bearing axially disposed between the first impeller and the first motor, and the second magnetic bearing includes a second impeller side radial magnetic bearing axially disposed between the second impeller and the second motor.
 8. The centrifugal compressor according to claim 7, wherein the first magnetic bearing includes a first remote side radial magnetic bearing axially disposed on a side of the first motor that is remote from a side where the first impeller is mounted, and the second magnetic bearing includes a second remote side radial magnetic bearing axially disposed on a side of the second motor that is remote from a side where the second impeller is mounted.
 9. The centrifugal compressor according to claim 8, wherein the first magnetic bearing includes a first axial thrust magnetic bearing, and the second magnetic bearing includes a second axial thrust magnetic bearing.
 10. The centrifugal compressor according to claim 8, wherein the first axial thrust magnetic bearing is axially disposed adjacent to the first remote side radial bearing, and the second axial thrust magnetic bearing is axially disposed adjacent to the second remote side radial bearing.
 11. The centrifugal compressor according to claim 6, wherein the first shaft has a first inlet end with the first impeller mounted thereon and a first remote end with the first motor mounted on the first shaft between the first impeller and the first remote end, the second shaft has a second inlet end with the second impeller mounted thereon and a second remote end with the second motor mounted on the second shaft between the second impeller and the second remote end, the first magnetic bearing includes a first remote side radial magnetic bearing axially disposed on a side of the first motor that is remote from a side where the first impeller is mounted, and the second magnetic bearing includes a second remote side radial magnetic bearing axially disposed on a side of the second motor that is remote from a side where the second impeller is mounted.
 12. The centrifugal compressor according to claim 6, wherein the first magnetic bearing includes at least one first radial magnetic bearing and at least one first axial thrust magnetic bearing, and the second magnetic bearing includes at least one second radial magnetic bearing and at least one second axial thrust magnetic bearing.
 13. The centrifugal compressor according to claim 12, wherein the first axial thrust magnetic bearing is axially disposed at the first remote end, the second axial thrust magnetic bearing is axially disposed at the second remote end, and the first and second remote ends and the first and second axial thrust magnetic bearings are spaced from each other to form a gap therebetween.
 14. The centrifugal compressor according to claim 1, wherein the first diffuser is connected to the second impeller such that refrigerant compressed in the first compression mechanism is further compressed in the second compression mechanism.
 15. A chiller system including the centrifugal compressor according to claim 1, the chiller system further comprising: an evaporator; a condenser; and an expansion device, the compressor, the evaporator, the condenser and the expansion mechanism being connected together to form a refrigerant circuit.
 16. A chiller system according to claim 15, further comprising: an economizer connected between the first compression mechanism and the second compression mechanism in the refrigerant circuit. 